This invention relates to an illumination system and a scanning exposure apparatus using the same. The present invention is suitably applicable to a projection exposure apparatus, particularly, a scanning exposure apparatus, for use in a lithographic process among production processes for producing various devices such as ICs, LSIs, CCDs, liquid crystal panels, or magnetic heads, for example, for transferring, by projection exposure, a circuit pattern of an original such as a photomask or a reticle (hereinafter, xe2x80x9creticlexe2x80x9d), being uniformly illuminated, onto a wafer while scanning the reticle and the wafer in synchronism with each other.
As a microprocessing technology for semiconductor devices such as ICs or LSIs, Japanese Laid-Open Patent Applications, Laid-Open No. 28313/1995, No. 190966/1997, No. 167735/1997, and No. 172901/1998 show scanning exposure apparatus for forming an image of a circuit pattern, formed on a reticle, upon a wafer (photosensitive substrate) through a projection optical system, while scanning the reticle and the wafer in synchronism with each other.
In this type of exposure apparatus, a reticle and a wafer are scanningly moved relative to a slit-like exposure area, by which one shot area on the wafer (and the whole pattern region defined on the reticle) is exposed. After the scanning exposure of one shot is completed, the wafer is moved stepwise to a next shot exposure position, and the scanning exposure of the next shot is initiated. This operation is repeated until exposures of the whole wafer are completed.
In accordance with recent miniaturization of a semiconductor device, the exposure wavelength is made shorter and shorter. Thus, as regards light sources to be used for the exposure, a KrF excimer laser (emission wavelength 248 nm) and an ArF excimer laser (emission wavelength 193 nm) as well as an F2 excimer laser (emission wavelength 157 nm) have to be taken into account.
The miniaturization of a semiconductor device is a largest factor for supporting the dynamics of the semiconductor industry. The required linewidth has changed rapidly, from a generation requiring resolution of a linewidth of 250 nm (256 MB DRAM) to generations requiring a linewidth of 180 nm, to a linewidth of 130 nm, and to a linewidth of 100 nm.
In the lithography up to the i-line (wavelength 365 nm), resolution finer than the exposure wavelength has not been carried out. However, in the lithography using a KrF excimer laser, although its wavelength is 248 nm, it is applied to the resolution of a linewidth of 180 nm and to 150 nm. It can be said that the resolution less than the exposure wavelength has to be practiced by all means, including advancements in resist materials and super resolution technologies, for example. When various super resolution technologies are used, a linewidth corresponding to a half wavelength, in terms of lines-and-spaces, will be practicable.
However, the super resolution technologies involve many restrictions in dependence upon a circuit pattern formed on a reticle. The most effective way to improve the resolving power is to shorten the exposure wavelength and to enlarge the numerical aperture (NA) of a projection optical system. This fact generates a large motivation to shortening the wavelength, and it leads to development of lithography using an F2 excimer laser.
When the exposure wavelength is to be shortened to improve the resolving power, for the exposure wavelength region shorter than 200 nm, there is a large limitation with respect to usable optical materials, and there arises a problem that the efficiency of light utilization becomes extraordinarily poor.
When an ArF excimer laser is used as a light source, optical materials usable in the region of that emission wavelength are only quartz and fluorite. When an F2 excimer laser is used as a light source, only fluorite is a usable optical material. Further, while these materials are usable, there is another problem. That is, for example, while fluorite has a transmission factor of 99.9%/cm or more with respect to the emission wavelength of an ArF excimer laser, even a best sample may show a value of only 99.5%/cm to 99.6%/cm with respect to the emission wavelength of an F2 excimer laser.
Situations are similar in regard to films (optical thin films). In the emission range of an F2 excimer laser, use of an oxide is almost impossible, and usable materials are limited only to fluorine series compounds. As for materials of low refractive index, there are only MgF2 and AlF3. As for those of high refractive index, there are only LaF3 and GdF3, for example. Therefore, with respect to an anti-reflection film, for example, a film having attained a transmission factor of about 99.7% will obtain a transmission factor of 99%, at the best.
The performance of a film is an important factor for determining the overall efficiency of lithography, using an F2 excimer laser. If it is assumed, for example, that there are ten transmissive or reflective surfaces until the light from a laser impinges on a wafer surface, the efficiency per one surface differs between 99% and 98%, for example, as can be readily understood from the relations 0.99100=0.366 and 0.98100=0.133, the difference of 1% results in a total difference of 2.5 times more.
However, in the emission range of an F2 excimer laser, because of the material limitations or a difficulty in surface treatment, basically, the performance of a film is not comparable to that when a KrF excimer laser or an ArF excimer laser is used. If a good film to be used with an F2 excimer laser is produced, the result can be reflected and a film of better performance can be produced for KrF and ArF excimer lasers.
As regards the film formation for use with an F2 excimer laser, therefore, it is quite important to produce a film having the same performance as that of currently available films used with conventional excimer lasers. Also, it is very important to produce a film having a durability to F2 excimer laser light.
Particularly, in an exposure optical system of an exposure apparatus, an illumination optical system includes more optical components than a projection optical system, and thus, the former is a key to the efficiency of light utilizations. While the projection optical system has a single function for printing an image of a reticle on a wafer without distortion, the illumination optical system is a multiple-function system having a shaping function for transforming light from a light source into an appropriate size, an integrating function for providing uniform illumination, an additional function for accomplishing various illumination modes, an imaging function for the masking, a function for controlling the light quantity, for example, and so on.
For a better light utilization efficiency of the illumination optical system, the number of optical components should be reduced as much as possible, and each constituent element should be designed to have multiple functions, or it should be simplified. On the other hand, the illumination optical system must meet requirements from the projection optical system, peculiar to the lithography using an F2 excimer laser.
As an example of such requirements, when the projection optical system is a catadioptric system (an imaging optical system having a combination of mirrors and lenses), there may be cases in which, depending on the structure of the optical system, it is required to attain illumination corresponding to an imaging region of an arcuate shape. In other words, the illumination optical system is required to produce a slit-like illumination region of an arcuate shape.
As regards the catadioptric system, the optical material for a projection optical system which is usable in the emission wavelength range of an F2 excimer laser is only one, i.e., fluorite. Therefore, with an ordinary dioptric system, chromatic aberration cannot be corrected. For this reason, the catadioptric system will be a good choice for a projection optical system to be used with this emission wavelength range. It should be noted here that a catadioptric system itself does not directly meet a slit-like illumination region of an arcuate shape. Depending on the structure of an optical system, a slit-like region of an oblong shape may be defined.
It is an object of the present invention to provide a good-efficiency optical system, so as to meet at least one of the strict requirements in relation to the efficiency of light utilizations, involved in the lithography using an F2 excimer laser.
It is another object of the present invention to provide an illumination system and a scanning exposure apparatus, by which the number of optical components can be reduced and by which various illumination modes as well as slit shapes can be met.
In accordance with an aspect of the present invention, there is provided an illumination system, comprising: a hologram; an optical system for projecting light from a light source to said hologram; slit means disposed at a predetermined position where slit-like light is formed by said hologram or at a position adjacent thereto; and an imaging optical system for illuminating a surface to be illuminated, by use of light passing through a slit of said slit means.
In one preferred form of this aspect of the present invention, said imaging optical system may serve to image the slit of said slit means, upon the surface to be illuminated or at a position adjacent thereto.
The hologram may be disposed perpendicularly to an optical axis of an optical system following said hologram.
The illumination system may further comprise an axicon for changing the shape of light impinging on said hologram.
The illumination system may further comprise a pyramidal prism for changing the shape of light impinging on said hologram.
The illumination system may further comprise an axicon and a pyramidal prism for changing the shape of light impinging on said hologram, wherein said axicon and said prism may be inserted into or retracted out of a light path in accordance with an illumination condition.
The optical system may have a zoom lens for changing the size of light impinging on said hologram.
Parallel light may impinge on said hologram.
The illumination system may further comprise a photoelectric detecting element for receiving zeroth order light from said hologram.
The optical system may have an oblique incidence correcting optical system for causing light to be obliquely incident on said hologram and for correcting a lateral-to-longitudinal difference of effective light upon said hologram due to the oblique incidence.
The imaging optical system may have a Dyson optical system.
The shape of the slit may be oblong or arcuate.
The slit means may have a first light blocking blade disposed at a position optically conjugate with the surface to be illuminated, and a second light blocking blade disposed at a position shifted from the optically conjugate position in an optical axis direction.
The illumination system may further comprise oscillation means for oscillating said hologram.
The slit-like light may be formed by passing diffraction light from said hologram through a Fourier transform lens.
The Fourier transform lens may comprise a telecentric system.
The Fourier transform lens may be arranged so that a portion of, or the whole of, the same is movable along an optical axis direction.
In accordance with another aspect of the present invention, there is provided an illumination system for use in an exposure apparatus, characterized by a hologram effective to define a slit-like illumination area upon a surface to be illuminated, through or without an optical system.
In accordance with a further aspect of the present invention, there is provided a scanning exposure apparatus, characterized in that a reticle is placed on a plane which is to be illuminated by an illumination system as recited above, that the reticle is illuminated with the illumination system, and that a pattern formed on the reticle is transferred by projection exposure onto a wafer through a projection optical system while the reticle and the wafer are scanningly moved in synchronism with each other.
In accordance with a yet further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: coating a wafer with a photosensitive material; transferring, by projection exposure, a pattern formed on a reticle onto the wafer by use of a scanning exposure apparatus as recited above; and developing the photosensitive material on the exposed wafer.
An illumination system or a scanning exposure apparatus using the same, to be described below, uses a hologram for producing illumination light having a slit-like sectional shape. Also, there is an optical system in which the light to be incident on the hologram is convergent light, being converged toward a xe2x80x9cparticular pointxe2x80x9d, and in which the sectional shape of the convergent light is controlled into a desired shape.
Here, if the xe2x80x9cparticular pointxe2x80x9d is infinite, the light incident on the hologram is parallel light. If, on the other hand, it is a virtual point, the incident light becomes apparently divergent light. Anyway, it is important that a particular point is fixed.
Among them, one which simplifies the structure is the case of parallel light. Handling the light in the form of parallel light leads to a reduction in the number of optical components. Also, it is advantageous with respect to the characteristic of a coating film.
A hologram has a property that, when light incident thereon is light being converged toward a xe2x80x9cparticular pointxe2x80x9d, an image can be produced even if only a portion of the incident light is used. Thus, by changing the sectional shape of the incident light to a desired shape, an illumination optical system which causes small degradation of illuminance in response to a change in illumination mode, can be accomplished.
An example of an illumination system of an exposure apparatus using a hologram is disclosed in Japanese Laid-Open Patent Application, Laid-Open No. 176721/1999, filed by the same assignee of the subject application.
A hologram may be used to produce a desired distribution such as a ring illumination zone, at the position of an optical integrator, and it may function to control the light quantity distribution to be produced at the position of a pupil of a projection optical system.
In one embodiment, the formation of a slit-like shape to be used for the exposure is accomplished by using a shape converting function of a hologram itself. For conversion of the beam shape at a pupil position, separate means such as a pair of axicons, to be described later, or the like, is used.
As a hologram, a CGH (Computer Generated Hologram) which can be produced by computing diffraction patterns by using a computer, is used. As regards the slit shape to be produced, it is not a pattern aiming at the resolution limit, but a simple pattern of an oblong shape or arcuate shape, of the millimeter order. Thus, it can be produced easily by calculations.
As regards the type of hologram, both a transmission type and a reflection type are usable. As for the structure of a reflection type, the same structure is attainable regardless of the wavelength, only by changing a coating film. Therefore, versatile disposition is attainable. On the other hand, the transmission type shows a better transmission factor, and a CGH made of fluorite may be preferable for the lithography using an F2 excimer laser.
As regards the control of the shape of light incident on a hologram, a combination of a zoom optical system with an optical system having a pair of axicons will provide good efficiency. A system or systems corresponding to various beam shapes can be constructed.
With the structure described above, the illumination system has a flexibility to meet various illumination modes, and it effectively overcomes the problem of efficiency, which is very important in the lithography using an F2 excimer laser.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.